A magnetic resonance imaging (MRI) apparatus is a medical-use diagnostic imaging system that generates nuclear magnetic resonance in hydrogen nuclei within any plane traversing a test subject and that performs tomographic imaging within the plane, based on nuclear magnetic resonance signals being generated. In general, a slice gradient magnetic field is applied for identifying an imaging plane, simultaneously with providing exciting pluses that excite magnetization within the plane. Accordingly, nuclear magnetic resonance signals (echoes) are obtained, which are generated at a stage of convergence of magnetization that has been excited. In addition, a phase encoding gradient magnetic field and a readout gradient magnetic field, being orthogonal to each other within the tomographic plane, are applied for providing the magnetization with positional information, during a period from the excitation until obtaining the echoes.
The pulses for generating echoes and each of the gradient magnetic fields are applied according to a predetermined pulse sequence. Various pulse sequences are known depending on purposes.
In those kinds of pulse sequences, in general, the gradient magnetic field in the trapezoidal waveform is turned on and off at high speed, and therefore, extremely loud sound, from 80 dB to 100 dB or larger, is produced within a bore. This sound has loudness considerably jarring the test subject placed in the bore, even though the test subject wears headphones or earplugs. Since this type of sound becomes louder as a magnetization level becomes higher, countermeasures are needed against a high magnetic-field machine of 3 T (tesla) or higher.
As one of sound reduction techniques, there is suggested a technique for varying the shape of gradient magnetic fields (see Non Patent Document 1, Non Patent Document 2, and the like). In general, sound produced by the gradient magnetic field is expressed by a product of a frequency response function (FRF) inherent to the device and a distribution of frequencies of the gradient magnetic field waveform. It is known that sound becomes smaller at a frequency having a small FRF value (Non Patent Document 1). Since the sound becomes extremely small, when the FRF value is 200 Hz or lower, there is disclosed a technique using a low-pass filter to suppress a frequency component of the gradient magnetic field waveform, in a range where the FRF exceeds that level, thereby reducing the sound. Specifically, for the sound reduction, it is suggested to allow the gradient magnetic field having a trapezoidal waveform, to pass through the low-pass filter, so as to smoothen the variation of strength at a rise time and a fall time of the wave.
It is further suggested in the Non Patent Document 2 that a readout gradient pulse and a phase encoding gradient pulse have sine waveforms.
The Non Patent Document 3 discloses an ultrashort echo time imaging technique aiming at producing almost no sound. This technique employs a radial type three-dimensional imaging method, using neither a slice selective gradient magnetic field nor a phase encoding gradient magnetic field, and varies strength of the remaining readout gradient magnetic field step-by-step, thereby eliminating on and off of the gradient magnetic field and suppressing sound production.